1. Field of the Invention
The present invention is directed to a contour collimator for shaping a radiation beam in radiation therapy, of the type having two sets of individually displaceable lamellae disposed opposite each other, which can be clamped in place in selected positions within a holder.
2. Description of the Prior Art
Contour collimators are utilized for limiting the radiation field of ionizing radiation, such as for oncological radiation therapy, so that the distribution of the radiation dose can be matched to the shape of the target volume, which is usually irregularly shaped, particularly when treating a tumor. A contour collimator is disclosed in European Application 0 387 921 wherein the individual lamellae of two lamellae sets disposed opposite each other can be positioned with a mechanical device so that a radiation field of the desired shape is achieved. In this known apparatus, an adjustment means, for example an electric motor or a user-operable wheel, can be successively coupled to all of the individual lamellae, with the lamellae which are not momentarily coupled to the motor or wheel being locked in place. The shape of the radiation field set by displacing the lamellae can thus be achieved automatically and in a simple manner by actuation of the motor or the wheel. The overall apparatus, however, is relatively complicated and bulky. Moreover, the precision width which a desired shape can be achieved is limited by the thickness of the individual lamellae, with a finer adjustment of the contour being possible with thinner lamellae. In this known apparatus, as the lamellae become increasingly thinner, it is more difficult to couple only one lamella at a time to the displacement mechanism. This known apparatus, therefore, is primarily suitable for larger collimators.
Collimators are also known which are exclusively manually operable, which also include oppositely disposed sets of individually displaceable lamellae. A form matched to the desired radiation field, such as form matched to the shape of a tumor to be treated, is first produced, and is placed into the contour collimator between the sets of lamellae, with the lamellae completely retracted from the path of the radiation beam. The lamellae are then manually pushed against the form and fixed in place. This procedure, however, is time consuming, and it must be assured that all lamellae are positively made to abut the form, in order to avoid harmful irradiation of tissue surrounding tumor. Moreover, given extremely thin lamellae, there is the risk of the operator being cut when manually displacing the lamellae.
A contour collimator is disclosed in U.S. Pat. No. 4,754,147 for radiation therapy having individually displaceable rods disposed opposite each other. Adjustment of the individual rods takes place with the use of form elements at the rod ends, with the collimator being turned so that the individual rods fall against the form due to the force of gravity. The rods are then clamped in place. Adjustment of collimator elements using the force of gravity, however, creates problems because jamming can occur, particularly given lightweight collimator elements. Moreover, the necessary rotation of the collimator requires complicated mechanical devices, and manipulation of the collimator thus also becomes complicated.